There are several optical methods that can be used for detection of sound from a distance; the most commonly used being the technique which employs the concept of Optical interferometer, which is well known in the field of electro-optics. The technique involves a high powered coherent light source, like a laser; good quality optical accessories, like beam splitters, mirrors, collimating lenses etc.; and a photo detector. Although, the use of a laser to transduce sound from one place to another is well realized in controlled laboratory conditions, the standard existing interferometer techniques have many limitations if one were to adapt for real world practical applications.
Most of the devices which utilize optics as a means for monitoring acoustic vibrations, work on the well-known principle of Michelson's interferometer, which requires a sufficiently high powered coherent light (laser) source where the object and reference light beams need to be precisely aligned and only 25% of the intensity of light used reaches the detector plane where fringes or images form. In brief, two aligned beams of laser light, of which one beam is slightly delayed in relation to the other beam of same frequency, will cause the two beams to reinforce each other if they are in same phase or cancel each other if one beam is 180° out of phase. If one of the two beams is reflected by an object in motion such that the direction of motion is generally in the same direction as the non-reflected stationary beam, and the two beams are aligned by means of suitable mirrors into a single beam, the resulting interference pattern will move at a velocity that is twice the velocity of the moving object along the axis of the aligned beam. As the interference pattern moves in the direction of the aligned beam, a light sensor placed in the path of the beam will sense light intensity variations that vary as a function of the movements of the reflecting object. The interference caused by the beams of light has been used by inventors to construct microphones that are very sensitive and have other qualities. (U.S. Pat. No. 3,470,329 by N. O. Young, Sep. 30, 1969, entitled ‘Interferometer microphone’; U.S. Pat. No. 1,709,762 by V. K. Zworykin, Apr. 16, 1929, entitled ‘Interferometer microphone’; U.S. Pat. No. 4,479,265 by R. P. Muscatell, Oct. 23, 1984, entitled ‘Laser microphone’; U.S. Pat. No. 6,590,661 by J. M. Shnier, Jul. 8, 2003, entitled ‘Optical Methods for selectively sensing remote vocal sound waves’).
However, any quality difference in terms of aberrations, astigmatism, coma and distortion etc., results in substantial increase in the noise in the interference fringes produced at the detector plane thus affecting the practical realization of the technique for many applications. Ideally both legs of an interferometer should be of equal length. If the two jointly arriving beams are not phase synchronized, the constructive and destructive interference is degraded, thus limiting the device's sensitivity. Moreover, one of the practical problems in the techniques of the prior art is that the large optical path lengths involved in real world applications, make it extremely difficult to maintain equal path lengths for the interferometer.
FIG. 1 schematically illustrates the concept of generating an optical diffraction pattern [5] for monochromatic light, by a prior art optical diffraction method. One means of producing such a pattern is through the use of a plane wave [1] from a laser and an opaque object [2]. According to the established theories of optics, the sharp edge of the object [2] casts a shadow having a fairly sharp outline of the same shape as the object. However, the edge of this geometric shadow is not absolutely sharp and when examined closely it shows a system of dark and bright bands in the immediate neighbourhood of the edge at a point [4]. The system of dark and bright bands comprises a diffraction pattern in a small region around the point [4]. The resulting diffraction pattern [5], on a screen [3], is typically shown for the purpose of illustration. This pattern is due to the diffraction of light around the edge of the object [2] and a result of interference between the direct and the diffracted light rays. The diffraction pattern is well known as the Fresnel diffraction pattern.
The concept of optical diffraction is not new; however the technique could not find as many applications as the technique of interferometer did, in the field of vibration monitoring due to the practical limitations involved in its implementation. Generally, devices based on optical diffraction require an optically opaque object to bend a part of the laser beam and need a separate recording setup for recording the diffraction pattern for analysis. Because of the high sensitivities involved in generating optical diffraction patterns, even laboratory experiments under controlled conditions fail to yield high repeatability, if utmost case is not taken to meet the various criteria necessary to yield a diffraction pattern. And in the real world conditions the efficacy of this technique becomes doubtful where the environmental conditions also keep changing. Therefore, there exists a need for new devices which employ simpler but effective methods for generation and detection of optical diffraction pattern that would function with good repeatability and durability even under changing environmental conditions.
The present invention is a new method for simultaneously generating and detecting the Fresnel diffraction pattern, as disclosed in the description. The new method of the invention can be easily adopted in making novel devices to monitor mechanical or pressure vibrations remotely; using any standard laser source and photo detector, one such example being the optical microphone for remotely detecting sound.
The present invention provides for methods and apparatus for sensing any vibrations, including sound waves; and in particular using optical means to detect any mechanical vibrations through certain corresponding changes in optical properties of air or through other optically transparent or semitransparent medium through which the mechanical vibrations, including sound waves, propagate. The present invention generally relates to the use of the principle of optical diffraction and interference.